1. Technical Field
The present disclosure relates to an erasing method adapted to a page-erasable EEPROM-type memory (Electrically Erasable Programmable Read-Only Memory).
2. Description of the Related Art
One of the main differences between a Flash-type memory and an EEPROM-type memory lies in the erase-granularity, i.e., the minimum number of memory cells likely to be erased during an erase cycle. Generally, the erase-granularity of an EEPROM memory is one page or one byte, whereas that of a Flash memory is one sector of several pages the number of which depends on the memory architecture. Furthermore, the physical size of a memory cell is a significant factor distinguishing Flash- and EEPROM-type memories. The memory cell of a Flash-type memory is typically three or four times smaller than an EEPROM-type memory cell. However, to write a single word in a Flash memory it is necessary to re-program a full sector of the memory.
Several solutions have been developed to attempt to benefit both from the reduced size of Flash-type memory cells and from the reduced erase-granularity of EEPROM memories. Generally, these solutions involve emulating an EEPROM memory using a Flash memory. A classic solution includes using a random access memory (RAM) to store a sector in which a word must be written. This solution implements a mechanism of writing a word, configured to load into the random access memory the sector containing the address of the word to be written, and the word to be written at this address, to control the erasing in the Flash memory of the sector loaded into the random access memory, and to transfer the sector from the random access memory to the Flash memory. This solution has also been adapted to solely page-erasable EEPROM memories, which correspond to one line of memory cells of the memory, to reduce the erase-granularity to the size of a word.
This solution has several disadvantages. It requires relatively large additional circuits, comprising a management circuit and a random access memory. It triggers cycles of erasing and programming not only the memory cells to be changed, but also all the other memory cells of the accessed sector. The result is a high consumption of energy and time compared to the writing of a single word in a word-erasable EEPROM memory. Given that a memory cell can only withstand a limited number of erase and program cycles, typically a hundred thousand cycles, this solution considerably reduces the service life of the memory. Furthermore, the two sector transfers between the Flash memory and the random access memory increase the risk of corrupting data.
It may therefore be desirable to combine the advantages of EEPROM memories, in particular in terms of erase-granularity, with the possibility offered by Flash memories of erasing extended to the scale of an entire sector.